Not necessarily. I don't see anyone under 21, because I would feel creepy if I did.
Otherwise, I kinda get why some people have a policy of seeing no one under 30. I have clients in their 20's who are lovely and amazing, but really often I've noticed that unfortunately younger clients tend to be more rude or pull no call no show's or ask for very very last minute appointments ''can i come see you in 30 minutes'' like no lol. I don't think it's ageism, it's just that I think a lot of guys in their 20's have a different lifestyle and may not be respectful or appreciative of a SP's time as much as guys who are a bit older.
I think part of it has to do with income as well, a lot of younger guys are students and can't afford to see an indy SP. When I charged much lower, my clients were often younger.
To tell you the truth, even guys in their 50's and 60's sometimes act like clowns and aren't serious about booking, so I'm making a generalization based on observations.